New monitor now operational in the Large Hadron Collider

A novel beam diagnostic instrument developed by researchers in the University of Liverpool’s QUASAR Group has been approved for use in the Large Hadron Collider (LHC), the world’s most powerful particle accelerator.

The new device, known as the Beam Gas Curtain (BGC) monitor, addresses one of the toughest challenges in modern accelerator physics: measuring the properties of very high-energy particle beams without disturbing them.

It has now been cleared for continuous operation (~2,000 hours per year).

The BGC monitor was conceived, developed, and refined over a decade within the QUASAR Group, part of the University’s Department of Physics and the Cockcroft Institute.

Professor Carsten P. Welsch, Head of the QUASAR Group, led the development of the technology for almost 20 years.

He said, “This is a tremendous achievement for our collaboration. Seeing a device that began as a concept explored by several generations of our Ph.D. students now operating at the heart of the LHC is truly inspiring. It demonstrates the power of long-term innovation, teamwork, and persistence.”

How the Beam Gas Curtain works

In a paper published in Physical Review Research, the Liverpool team and their GSI and CERN collaborators report the first-ever full-cycle, non-invasive beam emittance measurements at the LHC using the Beam Gas Curtain technique.

The instrument works by creating an ultra-thin, supersonic sheet of neon gas—a “curtain”—that interacts with the circulating proton or lead ion beam. The resulting faint flashes of fluorescence light are captured by a sophisticated optical system, revealing precise information about the beam’s size and quality throughout the full acceleration cycle.

Unlike existing instruments that require dedicated calibration time or interrupt normal operation, the BGC can continuously monitor the beam profile and emittance from injection at 450 GeV up to the LHC’s top energy of 6.8 TeV—all while physics experiments are running.

Testing, integration and future impact

The system was tested extensively at the Cockcroft Institute before installation at CERN. Its performance has exceeded expectations, delivering high-precision, non-invasive measurements for both proton and heavy-ion beams.

The Physical Review Research paper shows that the results agree closely with independent LHC diagnostics such as the Beam Synchrotron Radiation Telescope and emittance scans at the ATLAS and CMS experiments.

“Having our monitor now fully integrated into daily LHC operations is a real ‘wow’ moment,” said Dr. Hao Zhang, Deputy Group Leader in the QUASAR Group. “It is the culmination of years of development, from vacuum compatibility studies and optical design to software integration and on-site commissioning.”

With the BGC now approved as a permanent part of the LHC’s beam instrumentation, it paves the way for similar systems in other major research facilities, including the European Spallation Source in Sweden, the Electron Ion Collider in the U.S., and even medical accelerator applications.

“This achievement shows how university-based innovation can directly shape the tools that keep the world’s largest scientific instruments running,” added Professor Welsch. “It is a very proud moment for Liverpool and for all the students and researchers who contributed to this remarkable journey.”